A typical pulse-Doppler radar system emits a series of pulses, and further can collect echo signals. For each pulse, the echo signal(s) is correlated against the transmitted waveform to provide a range sounding, whereby the range soundings are compared against each other across pulses to discern Doppler information. The correlation function may be implemented as an equivalent matched filter, or as a direct correlation.
Radar modes that typically operate in this fashion include Synthetic Aperture Radar (SAR), Inverse-SAR (ISAR), various Moving Target Indicator (MTI) radars, radar systems which are generically range-Doppler radars, coherent search radar systems, etc.
The choice of waveforms to utilize can depend on the objectives of the radar system with respect to ease of waveform generation, downstream processing issues, and desires for probabilities of detection, interception, spoofing, etc. Typically waveforms are required that offer a large time-bandwidth product to afford both high energy and wide bandwidth for improved range resolution. There are a plethora of waveforms from which to choose. These include, but are not limited to, Linear-Frequency-Modulated (LFM) chirp, Non-Linear FM (NLFM) chirp, stepped frequency systems, various phase-coded modulation schemes, and random and pseudo-random noise waveforms. Each has its own advantages and disadvantages.
A conventional radar system employs a final transmit power amplifier that is normally operated in compression to maximize transmitted power output and/or efficiency. However, such operation of the amplifier is non-linear and, accordingly, may not faithfully reproduce amplitude modulations, etc., and further, may act to limit such modulations.
It is well-known to a person having skill in the art that the output of a matched filter, when input with a signal to which it is matched, is the autocorrelation function of the waveform. Furthermore, the autocorrelation function is related by the Fourier Transform (FT) to the Energy Spectral Density (ESD) of the waveform. That is, the autocorrelation function and ESD are FT-pairs. Matched filters have a principal advantage of maximizing the Signal-to-Noise Ratio (SNR) of energy in a final range-Doppler map. Most radar processing seeks to implement matched filters, or at least nearly so.
A problem with matched filters for many waveforms is undesirably high processing sidelobe levels in a range-Doppler map. The high processing sidelobe levels are usually mitigated with additional filtering, often by using data tapering, or window functions, during the processing. Although this may ‘un-match’ the filter to a degree, which can result in a slight degradation of the range-Doppler map SNR, this tradeoff is usually deemed worthwhile. While the SNR degradation is termed as “slight”, it is typically in the 1-2 dB range.
An alternative is to use waveforms designed to exhibit desirable ESD properties, where the autocorrelation of the waveform exhibits desirable, or at least acceptable processing sidelobe levels directly, that is, without additional filtering and the attendant SNR loss.
Various techniques relate to noise radar (radar that uses noise as a transmitted waveform), however, most techniques fail to address issues relating to sidelobe suppression. Where sidelobe suppression is addressed, a conventional technique operates on received data only, by reducing SNR, or operates in a non-linear fashion with resultant adverse effects to some subsequent exploitation schemes. Other techniques relate to shaping the ESD, but typically, such techniques may be limited to specific functions like a Gaussian distribution of frequencies to reduce processing sidelobes in a SAR image.